Low loss optical fiber designs and methods for their manufacture

ABSTRACT

The specification describes an improved optical fiber produced by a hybrid VAD/MCVD process. The core of the fiber is produced using VAD and the inner cladding layer has a depressed index and is produced using MCVD. In preferred embodiments, the optical power envelope is essentially entirely contained in VAD produced core material and the MCVD produced depressed index cladding material. Optical loss is minimized by confining most of the optical power to the VAD core where OH presence is low, as well as by maximizing the optical power in the un-doped silica region. The MCVD substrate tube material is essentially devoid of optical power.

RELATED APPLICATION

This application is a continuation of application Ser. No. 12/381,302,filed Mar. 10, 2009, which is a division of application Ser. No.11/170,257 filed Jun. 6, 2005.

FIELD OF THE INVENTION

This invention relates to a family of designs for optical fibers andmethods for producing optical fibers employing those designs havingimproved optical transmission characteristics. More specifically itrelates to a hybrid production method wherein optical fiber preforms arefabricated using a combination of MCVD and VAD techniques, wherein theVAD-MCVD interface is inside the waveguide-forming region of therefractive index profile design.

BACKGROUND OF THE INVENTION

A wide variety of methods have been proposed and explored for producingoptical fibers. As optical fiber technology has matured, three mainproduction methods, MCVD, VAD, and OVD have emerged. All involve thedeposition of glass particulates (frequently referred to as “soot”) on astarting substrate, then consolidating the particulates into a solidglass body. The techniques involve producing the particulates using anin situ vapor phase reaction. The vapor phase reaction is induced usinga torch, and the flame of the torch is directed at the startingsubstrate. In the MCVD method, the torch is directed on the outside of aglass starting tube, and the glass precursor gases are introduced intothe interior of a glass tube. The particulates are deposited on theinside surface of the tube. In the VAD and OVD methods, the torch andprecursor gases are directed onto the outside surface of a starting rodand the particulates are deposited on the end or side of the rod,respectively. Each technique is highly effective, and widely practiced.Each has well known advantages over the other.

For producing very high quality central core and inner claddingmaterial, the MCVD process would appear ideal. In the MCVD technique,the particulate layer grows incrementally in a radial direction. Due tothis incremental radial growth, MCVD is capable of producing morecomplex refractive index profiles than the VAD method. Complex indexprofiles are produced by changing the radial composition of theparticulate layer for each feature of the profile. Additionally, complexindex profiles frequently have one or more features with a depressed(relative to pure silica) index. Depressed index regions are usuallyformed by doping the particulates with fluorine. As will be described inmore detail below, the inside tube deposition method (MCVD) is moresuitable for fluorine doping than the either of the outside rod methods(VAD or OVD).

However, the need for using a starting tube can be a limiting factor inthe MCVD method. One limitation is when the glass in the MCVD startingtube is not of sufficient quality and low loss for large, state of theart, preforms (where some fraction of the optical power would be carriedby the starting tube material). If the initial tube quality limitationis avoided by the use of ultra pure (and typically expensive) materialto fabricate the starting tube, the exposure of the tube to theoxy-hydrogen torch typically used in MCVD as a heat source maycompromise the effective starting tube quality by the addition ofhydroxyl ions to a significant tube depth. Finally, the desiredrefractive index profile may require a dopant level in the regionprovided by the starting tube glass that is not compatible withsuccessful MCVD processing (viscosity, tube stability or heat transferconsiderations).

In VAD methods, the silica soot deposits and grows axially from astarting bait rod. A significant advantage of the VAD technique is thatit can be practiced as a continuous process. This allows in-linedeposition, purification, drying, and sintering. After deposition iscomplete, the starting rod is separated from the deposited body and theentire preform, unlike conventional MCVD, may thus be made ofCVD-deposited material. As a general proposition, VAD methods areeffective and widely practiced, but they still do not match the abilityof MCVD to control precisely the radial deposition of index changingdopants, and thus the radial refractive index profile. Because of this,VAD methods and other soot deposition/subsequent sintering methods suchas Outside Vapor Deposition (OVD) are limited in the complexity of thefiber designs that can be efficiently produced. Moreover, the VAD methodis not well adapted for fluorine doping. This is especially the case forin-line VAD processes.

The recognition in the prior art that in a single mode optical fiber thecore and inner cladding together carry greater than 95% of the opticalpower but typically comprise less than 5% of the fiber mass, hasresulted in manufacturing processes that focus special attention on thefabrication of this region. Methods have evolved in which the core andinner cladding regions of the preform are produced by a relativelyadvanced and expensive method, while the outer cladding, the bulk of thepreform, is produced by a less demanding, less expensive, process. Theintegration of the core rod and the cladding is carried out in anovercladding process. The overcladding process is described generally inU.S. Pat. No. 6,105,396 (Glodis et al), and PCT/EPT00/02651 (25 Mar.2000), which are incorporated herein by reference for details of thegeneral techniques.

The overcladding process overcomes some of the limitations in thecomplexity of preforms produced by the VAD technique. Overcladding mayinvolve multiple overcladding tubes, each adding a distinct claddingregion, to reach the desired complexity of the fiber refractive indexprofile.

A commonly used process of this type is the so-called rod-in-tubemethod, where the core rod is made by a very high qualitydopant-versatile process, and the cladding tube is made of lessexpensive, lower purity, glass. In some cases, glass with a singlecomposition provides a low cost choice. In the rod-in-tube overcladdingprocess, the core rod is inserted into the cladding tube, and the tubecollapsed around the rod to form a unitary body. Again, multipleovercladding steps may be used, and in some cases one or more of thefinal overcladding steps may be combined with the fiber drawingoperation.

If a tube overcladding process is used, suitable cladding tubes may beproduced by soot deposition or extrusion of fused quartz. Making thesevery large cladding bodies with a soot based synthetic glass processleads to high quality glass but requires extensive processing and isrelatively expensive. Large bodies of fused quartz are less expensive,but are generally not of sufficient purity.

In summary, the VAD method when combined with the rod-in-tubeovercladding methods provides a rapid and economical method for forminglarge glass core rods with relatively simple index profiles. However,where the cladding comprises depressed index features, commerciallyavailable depressed index cladding tubes of the prior art do not providethe desired optical quality for the overall preform body.

STATEMENT OF THE INVENTION

We have designed a set of optical fiber index profiles that reduce thelevel of dopant related excess scattering loss, and have developed ahybrid method for producing optical fibers using a combination of MCVDand VAD that significantly advance the prior art in terms of loss andhigh productivity. The hybrid method combines the desirable features ofeach processing technique. We have also noted the loss characteristicsof optical fiber produced using these methods. Recognizing that the losscharacteristics are different, we have combined the MCVD and VADtechniques to optimize the composite loss characteristic. The profiledesign can be optimized to distribute the optical power so as to reduceRayleigh scattering, while achieving nearly zero water peak 1385 nm lossperformance and maintaining good macrobending performance. In thepreferred embodiment of the invention a VAD method is used to produce acore rod, with an up-doped inner core and a less heavily doped; orundoped, outer core. An MCVD method is used to produce a cladding withone or more depressed index regions. Using this basic preformfabrication approach, we adapt the profile and the optical powerenvelope such that essentially all of the optical power is contained ineither the VAD material and the MCVD material, and most of the powerenvelope is contained in the VAD material, with for example typically60% of the power contained in the VAD inner core, which has Ge-dopinglevels that are typically less than 60% of that found in standard singlemode fiber cores, and with for example 20-40% of the power contained inthe undoped or lightly doped VAD outer core.

A variation of this method is to substitute an ultra-high purity OVDtube for the MCVD cladding tube to produce a similar preform.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross section of an optical fiber showing anexample of a refractive index profile that may be made by the method ofthe invention;

FIG. 2 is a plot of the profile of FIG. 1;

FIGS. 3 and 4 are schematic representations of a rod-in-tube process;

FIG. 5 is a representation of an optical fiber drawing apparatus;

FIG. 6 is a plot of attenuation vs power weighted Ge and F concentrationfor various examples;

FIG. 7 is a plot of attenuation vs wavelength, often referred to as anattenuation spectrum for the optical fiber designated 63 in FIG. 6;

FIG. 8 is a plot of loss vs. power fraction showing the effect ofdifferent characteristics of a rod-in-tube interface.

DETAILED DESCRIPTION

Low loss fibers with low or zero water peaks are effectively produced bythe VAD or OVD methods in large economical preform sizes (greater than90 mm OD). The lowest 1385 nm losses (specified less than 0.31 dB/km andtypically 0.275 dB/km) are most consistently achieved with core rodsproduced by the VAD method, in preform sizes as large as 150 mm. TheMCVD method is typically used to produce smaller preforms (60 to 90 mm)with somewhat relaxed water peak specifications compared to VAD. Thiscommercial practice results from the fact that core material depositedby VAD or OVD is usually inherently more dry ([OH]<1 ppb) due to thedehydration of the soot prior to sintering. A second causative reason isthat it is possible to make large bodies with large D/d ratios with VADor OVD; this means that the first overcladding interface can be far awayfrom the core (i.e. low optical power) in a large core body (muchgreater than 90 mm). The MCVD process normally produces to lower D/dratios unless the preform size is small (<70 mm), making it moredifficult to achieve economical, low or zero water peak fibers in largepreforms.

Commercially produced fibers having dispersion at 1550 nm ofapproximately 17 to 20 ps/nm/km and a zero dispersion wavelength near1310 nm can generally be divided into two types: Ge-doped core withsilica cladding vs. silica core fiber with F-doped cladding. In eithercase, the total relative delta of the waveguide is approximately 0.35%.The former class of fiber is generally known as matched clad or standardsingle mode fiber, and have optical loss on the order of 0.185 to 0.195dB/km. The latter class is usually known as pure silica core fiber andhas very low commercial optical loss values near 0.168 dB/km, due to thelower Rayleigh scattering of pure vs. doped silica.

Although it has been demonstrated that silica core fiber has excellentloss characteristics, unfortunately, to achieve this level ofperformance requires draw speeds 5 to 10× slower than that for matchedclad fiber. This restriction on draw speed impacts factory productivityand throughput, which makes silica core fiber more expensive tofabricate. This has effectively prevented its widespread commercialadoption, rendering it a niche product for undersea or longun-repeatered single-span applications.

As will be discussed below, the class of index profiles of thisinvention addresses these two limiting aspects of the prior art. In theprior art, either 1) the lower limit of the fiber loss is determined bythe excess Rayleigh scattering level of the heavily Ge-doped core, aswith conventional Ge-doped SSMF, or 2) the practical application spacefor the fiber is limited by extra cost associated with the need to drawthe fiber at very low speed, as with pure silica core fibers. The classof index profiles in the current disclosure provides a means to achieveloss values that approach those of pure-silica fibers while maintainingthe ability to draw these fibers at speeds normal for standard singlemode fiber. Further, the hybrid VAD-MCVD process disclosed here is thepreferred method for fabricating this class of index profiles.

Referring to FIG. 1, a cross section of an optical fiber preform 11 isshown with doped regions 12-16. The regions represent five differentrefractive indices, in individual layers that extend radially from thecenter of the fiber. This profile is representative of the complex indexprofile discussed above. Regions 13 and 16 represent the index of puresilica, and regions 14 and 15 are down-doped.

The optical fiber core region 12 is relatively heavily doped, typicallywith GeO₂. A second core region, 13, is a more lightly doped region, andin a preferred example, is intrinsic. (In this discussion, the glassesare assumed to be silica-based glasses, and the term “intrinsic” refersto undoped silica.) Cladding region 14 is a depressed (down-doped)region. Cladding region 15 is a less heavily doped depressed region, andregion 16 is shown in this embodiment as an intrinsic region, butalternatively may be an extension of region 15.

Due to the presence of the depressed index region in this profile, themethod of choice for preparing the entire preform for the optical fiberwould typically be MCVD. This is because in the MCVD method it isrelatively straightforward to dope the depressed region 14 withfluorine. Fluorine doping is normally achieved by exposing sootparticles to SiF₄ (via introduction of SiF₄, SF₆ C₂F₆, etc., as known inthe art). This allows fluorine to diffuse into the porous glassstructure and diffuse into the surface of the particulates. Because MCVDsoot is deposited and sintered in a layer-by-layer fashion, the F doesnot diffuse away. Thus a relatively high level of F-doping as well asprecise control over concentration profile can be achieved. For moredetailed information on fluorine doping see co-pending application Ser.No. 09/755,914 filed Jan. 5, 2001.

This type of process may be incompatible with the some of the preferredVAD processes, especially those where the purification and consolidationsteps are performed on an entire boule after soot deposition. Thus MCVDis often the preferred choice for making preforms with depressed indexfeatures in the profile. These profiles are important for low dispersionslope optical fiber, as well other state-of-the art optical fiberproducts.

Although these factors point to the use of all MCVD for making preformswith depressed index regions, we have found that important differencesin the intrinsic levels of OH contamination in preforms made using VADvs preforms made using MCVD may alter that conclusion. The OH contentcan be related to losses in relevant transmission or Raman pump bands,especially the S wavelength band between 1460 and 1530 nm, as well as inthe 1385 nm window. The level of GeO and other lose producing defectsmay also be lower for VAD material with respect to MCVD material. Inboth cases we have measured better loss results for optical fiber drawnfrom VAD produced preforms. Accordingly, we have demonstrated theefficacy of producing the core region of an optical fiber preform, e.g.regions 12 and 13 of FIG. 1, using VAD, and the cladding regions 14 and15 using MCVD.

FIG. 2 shows a refractive index profile 21 for an optical fiber drawnfrom the preform represented in FIG. 1. Note that FIG. 1 represents apreform design profile (the preform OD is typically 63 mm), while thespecific profile in FIG. 2 is the index profile for the optical fiber.Typically, optical fiber produced from a preform essentially replicatesthe preform profile, but with smaller dimensions.

Regions 12-16 in the preform of FIG. 1, from which the optical fiber ofFIG. 2 is derived, are designated at the top of FIG. 2. The inner coreregion 12 is doped with Ge to produce an index at the core center ofdelta ˜ +0.002. Delta is the index deviation from the intrinsic index ofsilica. The ordinate in FIG. 2 is shown as an absolute index difference,but is often expressed as a percent by multiplying the values shown by100. The index level of inner core region 12 will be recognized by thoseskilled in the art as relatively low compared with conventional singlemode fiber, and can be achieved with less than 2 wt % Ge. In a typicalsingle mode core the core center has doping levels of 3.5 wt % or more.The relatively low doping level of the core in the optical fibers of theinvention reduces the optical loss in the fiber. The width of the innercore in this example is approximately 4 microns. The outer core region13 in this example is intrinsic silica. Alternatively, the outer coremay be lightly doped. It may be doped with Ge, for example, to a nominallevel of less than +0.001. In yet another alternative embodiment, it maybe lightly down-doped with fluorine. One objective is to produce anouter core layer with relatively low doping, thus resulting in a verylow loss core material. A delta range of −0.001 to +0.0005 is suitable.The width of the outer core region is shown with the same width as theinner core region, i.e. approximately 4 microns. The next region is adepressed index region 14 with an index delta of about −0.002 and awidth of about 8 microns. This region is produced using MCVD. The indexof refraction in this region is typically approximately constant as afunction of radius, but is not required to be flat. The depressed indexregion generally consists of SiO₂, doped with an appropriate amount offluorine to achieve the desired index of refraction. Region 15 in thisexample is formed by the MCVD starting tube. The starting tube isslightly down-doped to an index of approximately 0.008. The width ofthis region, approximately 16 microns, is determined by the thickness ofthe starting tube. The last layer shown in FIG. 2 is undoped region 16.It is convenient to form this with an undoped overclad tube. However, todecrease the sensitivity of the fiber to bending losses it mayalternatively comprise the same slightly down-doped material as region15 and still retain the low loss, high draw productivity features of theinvention. The optical transmission properties of this region arerelatively inconsequential since essentially no optical power travels inthis region.

The optical power envelope that represents the optical powerdistribution in the optical fiber example of FIG. 2 is shown at 22. Thevertical scale is arbitrary. The optical power is strongly guided in thecenter region of the inner core 12 and declines exponentially, with anessentially Gaussian shape, through the outer portion of the inner coreand through the outer core. At the interface between the core and thedepressed cladding region 14, the power has decayed to a low value. Onegoal of the invention is to confine essentially the entire powerenvelope to the low loss VAD and MCVD regions. At the interface wherethe MCVD starting tube begins, i.e. the interface between regions 14 and15, there is essentially no optical power. Thus the optical loss andother optical characteristics of region 15 are not as consequential asthose of the inner layers where essentially the entire nominal powerenvelope is contained. Under some circumstances, for example when thefiber is subject to bending, the outer regions come into play, and aidin preventing loss. Consequently, the design of the regions beyond theMCVD trench layer 14 may have different compositions and properties thanthose shown. For example, region 16 may be down-doped to control bendingloss. Or the MCVD starting tube may be doped to levels comparable tothat for region 14. Other regions may be included, for example one ormore ring regions of up-doped material.

Referring back to the power envelope 22 in FIG. 2, measured values ofthe relevant properties of this optical fiber is given in the followingtable. The power envelope for these data was measured at 1550 nm.

TABLE 1 Radius Region Index (delta) (microns) Power % Inner core 0.00184 60.3 Outer core 0.0 4-8 35.9 MCVD trench −0.002  8-16 3.8

Measurement of the power envelope at 1385 nm yielded a core powercontainment value of 97.8% and an MCVD layer power envelope ofapproximately 2.2%. As is well known, optical power decreasesexponentially throughout the structure so the power envelope containedin the VAD/MCVD combined regions will not be 100%, but with the designsof the invention will be at least 99%.

The goals of the invention are generally satisfied for optical fiberhaving the prescriptions in the following table:

TABLE 2 Region Index (delta) Radius (microns) Power % Inner core 0.003to 0.001 2-8  50-80 Outer core 0.001 to 0.000 3-10 20-40 MCVD region−0.0035 to −0.0007 5-25 <5

In preferred embodiments, the combined inner core and outer core radiusequals 5-12 microns.

As mentioned earlier, in the embodiment of FIG. 2 the width of thedown-doped region 14 may be 8 microns. In the case where this region 14extends to a radius of 25 microns (or less), as taught in Table 2 above,the radius of the inner core and outer core combined may extend out to17 microns (or less).

A feature of profiles with these general designs is that, in addition toconfining essentially all of the power envelope to the VAD/MCVD regions,the major fraction, >90%, and preferably >96%, is contained within theVAD regions, where both the OH content, and potential OH interfacecontamination, are small. The use of intrinsic silica, theoreticallyhaving optimally low loss, for a substantial portion of the core, andcarrying a substantial fraction of the optical power, contributessignificantly to the overall effectiveness of the design.

Preforms made with a VAD core rod and an MCVD cladding tube may beassembled using a rod-in-tube method. Typical rod-in-tube methods aredescribed in conjunction with FIGS. 3 and 4. It should be understoodthat the figures referred to are not necessarily drawn to scale. Acladding tube representative of dimensions actually used commerciallyhas a typical length to diameter ratio of 10-15. The core rod 32 isshown being inserted into the cladding tube 31. The tube 31 mayrepresent a single tube or several concentric tubes. The rod at thispoint is typically already consolidated. The tube may be alreadyconsolidated or still porous. Normally, there exist several commonoptions for the make-up of the core rod. It may be just the center core,or it may include one or more additional layers. In the main embodimentof the invention where the core rod is made using VAD the core consistsof layers 12 and 13. Cladding tubes made with very high qualityglass-forming techniques may be used for layer 14. However, in view ofthe limited availability of commercial available tubes with ultra-highpurity, layer 14 is preferably produced using MCVD. The down-doped layer14 is formed by a down-doped MCVD layer on the interior surface of asubstrate tube 15.

Referring to FIG. 4, after assembly of the rod 32 and tube 31, thecombination is collapsed to produce the final preform, where theinterface 34 between the outer surface of the rod and the inner surfaceof the tube is essentially indistinguishable. This step may occur eitherprior to or during the draw process.

Additional cladding operations, for example an added overclad tube forproducing layer 16, may follow essentially the same procedure as therod-in-tube method just described. Alternatively, the preform may beassembled by collapsing the overclad tubes(s), the MCVD tube, and thecore rod, in one operation.

The optical fiber preform, as described above, is then used for drawingoptical fiber. FIG. 5 shows an optical fiber drawing apparatus withpreform 51, and susceptor 52 representing the furnace (not shown) usedto soften the glass preform and initiate fiber draw. The drawn fiber isshown at 53. The nascent fiber surface is then passed through a coatingcup, indicated generally at 54, which has chamber 55 containing acoating prepolymer 56. The liquid coated fiber from the coating chamberexits through die 61. The combination of die 61 and the fluid dynamicsof the prepolymer controls the coating thickness. The prepolymer coatedfiber 62 is then exposed to UV lamps 63 to cure the prepolymer andcomplete the coating process. Other curing radiation may be used whereappropriate. The fiber, with the coating cured, is then taken up bytake-up reel 64. The take-up reel controls the draw speed of the fiber.Draw speeds in the range typically of 1-30 m/sec. can be used. It isimportant that the fiber be centered within the coating cup, andparticularly within the exit die 61, to maintain concentricity of thefiber and coating. A commercial apparatus typically has pulleys thatcontrol the alignment of the fiber. Hydrodynamic pressure in the dieitself aids in centering the fiber. A stepper motor, controlled by amicro-step indexer (not shown), controls the take-up reel.

Coating materials for optical fibers are typically urethanes, acrylates,or urethane-acrylates, with a UV photoinitiator added. The apparatus ofFIG. 5 is shown with a single coating cup, but dual coating apparatuswith dual coating cups are commonly used. In dual coated fibers, typicalprimary or inner coating materials are soft, low modulus materials suchas silicone, hot melt wax, or any of a number of polymer materialshaving a relatively low modulus. The usual materials for the second orouter coating are high modulus polymers, typically urethanes oracrylics. In commercial practice both materials may be low and highmodulus acrylates. The coating thickness typically ranges from 150-300micrometers in diameter, with approximately 245 micrometers standard.

The positive effects of the invention were demonstrated in the contextof known optical fiber loss mechanisms. The current state of art offabricating and designing silica based fibers has progressed to thepoint where in the 1550 nm region the excess loss from absorptioneffects such as contamination by OH or trace metals or the presence ofelectronic defects in the glass structure or from waveguide effects suchas bending losses have been essentially removed. Therefore, the lowerlimit of the loss of state of the art fiber is determined by theRayleigh scattering loss of pure silica. The presence of the indexchanging dopants that are used to form the waveguide structure, e.g.,typically Ge or F, has the detrimental effect of increasing the Rayleighscattering loss of the doped glass above that of pure silica. Ohashi(“Optical Loss Property of Silica-Based Single-Mode Fibers”, JLT Vol 10,No5, May 1992) determined that the Rayleigh scattering coefficient ofsilica fibers doped with Ge is:

A _(Ge) =A ₀(1+0.44*Δ_(Ge)),

and for silica fibers doped with F is:

A _(F) =A ₀(1+0.41*Δ_(F)),

where A₀ is the Rayleigh scattering coefficient of undoped silica,Δ_(Ge) and Δ_(F) are the normalized index differences associated withthe Ge and F doping, respectively. These formula clearly show thatsilica doped with germanium and fluorine has an elevated Rayleighscattering coefficient relative to that of pure silica, resulting inexcess scattering loss. The class of index profile designs disclosedhere are designed to have reduced levels of germanium and fluorinedoping in the regions of the fiber that propagate the most significantportions of the optical power, relative to matched clad fiber designs,and therefore provide a means to reduce the fiber loss by reducing theRayleigh scattering coefficient.

To quantify the effect on the fiber loss of the excess Rayleighscattering loss of the doped regions of the waveguide above that ofintrinsic silica, we define the excess scattering loss metric as theintegral over the fiber cross section of the Ge and F dopingconcentration weighted by the optical power density.

${{Excess}\mspace{14mu} {scattering}\mspace{14mu} {loss}\mspace{14mu} {metric}} \propto \frac{{∯{{{Ge}(r)}*{E^{2}(r)}{r}{\varphi}}} + {∯{{F(r)}*{E^{2}(r)}r{r}{\varphi}}}}{∯{{E^{2}(r)}r{r}{\varphi}}}$

where Ge(r) and F (r) are the Ge and F doping concentrations in units ofweight percent as a function of radial position, r, E (r) is theelectric field as a function of r of the fundamental mode.

FIG. 6 shows a plot of the observed loss of various large effectivearea, non-dispersion shifted fibers versus the excess scattering lossmetric. The points labeled 61 and 62 represent results for commerciallyavailable pure silica core and Ge-doped core fibers, respectively.Assuming that the fiber loss is dominated by Rayleigh scattering, thenfibers with excess scattering loss metric between the value of these twoexample fibers will have fiber loss that falls approximately along line65 which connects these two points. The points labeled 63 and 64represent the results for two examples of the inventive fiber. Thevalues of the excess scattering loss metric for these example fibers areslightly less than 2.0 and the measured loss values are about 0.180dB/km. The preferred embodiment of the invention has the index profileshown in FIG. 2 with the value of the excess scattering loss metric ofapproximately 1.0 and expected fiber loss of 0.174 dB/km. The range ofthe integral of the power weighted doping concentration for the presentinvention typically falls within the range of approximately 1 to 2.

FIG. 7 shows the attenuation spectrum of the fiber represented by point63 in FIG. 6, which was drawn at 18 m/s pulling speed. The attenuationof this fiber at 1550 nm and 1385 nm are 0.180 and 0.284 dB/km,demonstrating excellent low loss performance.

Another fiber loss performance issue that is related to the fiber indexprofile design and the fiber fabrication methods is the ability of thefiber to meet the Zero Water Peak (ZWP) properties. ZWP propertiesrequire very low initial loss at the OH absorption peak in thewavelength region around 1385 nm and stable long term aging loss whenthe fiber is exposed to molecular hydrogen over its operating lifetime.

An important aspect to providing ZWP properties is the extremely “dry”nature of the inner and outer core regions formed by the VAD depositionprocess (typically <0.5 ppb [OH]). In a preferred embodiment of theinvention, typically >95% of the optical power propagates within the VADformed regions of the fiber. Also important to providing ZWP propertiesis the very dry nature of the depressed index region adjacent to theouter core region. The dryness achievable with standard MCVD processing,[OH] concentration typically <3.0 ppb, provides sufficient performancefor this region since typically only a few percent of the powerpropagates within this region. It is foreseeable that realization ofthese fiber designs with ZWP properties will be possible by forming theentire depressed index trench region adjacent to the core rod with ultrahigh purity, F-doped tubing that has [OH] contamination level comparableto that of material formed by MCVD processing. However, since thecurrent state of the art of commercially available synthetic silicatubing results in [OH] contamination levels of about 200 ppb, thesetubes are not currently commercially available. Another attribute of theinvention in the context of ZWP performance is that more than 99% of thepower propagates within the regions formed by VAD, MCVD or ultra highpurity tubing. A fourth attribute of the invention in this context isthat the percentage of the optical power that is contained within the 1micron thick region centered about the interface between the VAD corerod and the first overcladding tube is less than about 2%, preferablyless than 0.5%. It is preferred that processing of the core rod prior toovercladding and the overcladding steps be consistent with maintainingthe average [OH] contamination across the 1 micron region centered aboutthe interface at 20 ppb or less. This may require processing techniquessuch as furnace stretching in a dry environment, plasma and/or chemicaletching of the rod and tube surfaces prior to overcladding and employinga drying agent such of chlorine to maintain a dry environment at theinterface void during the overcladding process. FIG. 8 shows acalculation of the excess loss at 1385 nm resulting from OHcontamination at the interface region as a function of the percentage ofpower propagating in the 1 um thick annulus centered at the interface.The two curves show the excess for the typical level of OH contaminationobserved when the dry overcladding processing described above and whenmore typical overcladding processing are used. For good ZWP properties,dry processing techniques are preferred, and when properly practicedshould result in “interface” power of less than about 2%.

The reduction of loss from 0.185 for best-in-class standard matched cladfibers to −0.175 dB for the invention disclosed here represents a 1 dBreduction in span loss for a 100 km terrestrial system, and −0.5 to 0.7dB for 50 to 70 km submarine system. This 1 dB in a terrestrial systemrepresents an additional dB of margin which can be used to extend totalsystem length or reduce the cost or specifications on other components.The zero (negligible) water peak loss extends the ability to place Ramanpumps near 1385 nm, and the combination of low Rayleigh scattering lossand low water peak loss makes Raman pumping more efficient across theentire region from 1350 to 1450 nm. For the submarine case theadditional 0.5 to 0.7 dB can be used to extend the distance betweenextremely expensive deep-sea repeaters, thus reducing the total numberof repeaters required to span a given system length. This representssignificant cost savings.

Fibers according to the present invention may be utilized withdispersion compensation modules based on negative dispersion fiber orhigher-order mode fiber. They may also be deployed as an element of adispersion-managed span, where the cabled fiber with positive dispersion(such as the present invention) is paired with a cabled fiber withnegative dispersion and negative dispersion slope. The design of adispersion-managed span usually places a segment of the positivedispersion fiber with large effective area immediately after thetransmitter, to minimize non-linearity when launch power is maximum. Thesegment of smaller effective area negative dispersion fiber is splicedinto the span after the optical power has been attenuated by fiber lossin the positive dispersion fiber to minimize non-linearity. For Ramanamplified systems, the sequence may be modified: a segment of largeeffective area, positive dispersion fiber; followed by a segment ofsmaller effective, area negative dispersion fiber; followed by a secondsegment of large effective area, positive dispersion fiber.

Unrepeatered (i.e. unamplified) systems are often desirable in remotelocations such as frontiers or between islands. In these systems, acombination of high launch power and distributed Raman amplificationhelp to enable transmission over 200-300 km without an in-line opticalamplifier. Such a system would have its transmission wavelength near theloss minimum of the fiber, usually around 1570 to 1580 nm. An opticalfiber of the type disclosed herein has reduced signal band attenuation,produced more economically than pure silica core fiber. It hasnegligible water peak loss, enabling placement of an advantageoussecond-order Raman pump at ˜1375 nm, extremely close to where the waterpeak would be. It also can have a large effective area greater than 100square microns to mitigate nonlinearities associated with higher launchpower.

The new class of optical fibers disclosed herein will also be ideal foran emerging category of 10 Gbps (and faster) transmission systems thatwill use some form of signal processing to mitigate intersymbolinterference due to chromatic dispersion. In these systems opticaldispersion compensation will either be supplemented by or will betotally replaced by pre-emphasis at the transmitter and/or equalizationat the receiver, with all signal processing done in the electricaldomain. These electronic dispersion mitigation schemes respond best tolinear, deterministic impairments. In this paradigm, the performance andcost of transmission systems will depend far less on mitigation ofdispersion impairments and more on reducing attenuation loss andnon-linearity. The category of fibers disclosed herein, such as with aneffective area of 110 um2 and signal band loss of 0.175 dB/km can becalculated to yield two dB of performance improvement over standardmatched clad fiber when used with such a system.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. An optical fiber for transmitting optical power in the 1550 nmwavelength region, the optical fiber having an index profile comprising:an inner core, an outer core having a minimum radius of 5 microns; atrench having a maximum radius of 25 microns; wherein the combinedradius of the inner and outer cores is in the range of 5-17 microns; andfurther whereby the index profile is shaped to confine: (i) 50-80percent of the optical power within the inner core, (ii) 20-40 percentof the optical power within the outer core, and (iii) less than 5percent of the optical power in the trench.
 2. The optical fiber ofclaim 1 wherein: the inner core has a maximum radius between 2-8microns; the outer core has a maximum radius between 5-17 microns. 3.The optical fiber of claim 1 where the inner core has an index deltabetween 0.001 and 0.003; the outer core has an index delta between 0.000and 0.001; and the trench has an index delta between −0.0035 and−0.0007.
 4. The optical fiber of claim 1 where the 1550 nm attenuationis less than 0.175 dB/km.
 5. The optical fiber of claim 1 where the 1383nm attenuation is less than 0.31 dB/km.
 6. The optical fiber of claim 1where the cable cutoff wavelength is less than 1260 nm.
 7. The opticalfiber of claim 1 where the cable cutoff wavelength is less than about1530 nm.
 8. The optical fiber of claim 6 where the 20 mm diametermacrobending loss is less than 2 dB/m.